Oxidative C(sp3)-H bond cleavage, C-C and CC coupling at a boron center with O2 as the oxidant mediated by platinum(II)

Article abstract of DOI:10.1039/c3cc47445c

Dimethyl- and diphenylplatinum(II) fragments Pt^IIR₂ (R = Me, Ph) enable facile and efficient oxidative C(sp³)-H bond cleavage and stepwise C-C and C=C coupling at the boron atom of a coordinated 1,5-cyclooctanediyldi(2-pyridyl)borato ligand with O₂ as the sole oxidant. the Partner Organisations 2014.

Full text of DOI:10.1039/c3cc47445c

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3
Q
Oxidative C(sp )–H bond cleavage, C–C and C C
coupling at a boron center with O2 as the oxidant
mediated by platinum(II)†
Cite this: Chem. Commun., 2014,
50, 5376
Received 29th September 2013,
Accepted 8th November 2013
S. Pal, P. Y. Zavalij and A. N. Vedernikov*
DOI: 10.1039/c3cc47445c
www.rsc.org/chemcomm
Dimethyl- and diphenylplatinum(II) fragments PtIIR2 (R = Me, Ph) enable
facile and efficient oxidative C(sp3)–H bond cleavage and stepwise C–C
Q
and C C coupling at the boron atom of a coordinated 1,5-cyclo-
octanediyldi(2-pyridyl)borato ligand with O2 as the sole oxidant.
The selective oxidation of C–H bonds using dioxygen, an inexpensive
and environmentally benign reagent, catalyzed by electrophilic
transition metals and pioneered by Hay1 and Shilov2 has recently
attracted much attention.3–7 In turn, organoboron compounds have
emerged as valuable nucleophilic partners in various oxidative
coupling reactions allowing for the formation of C–C and a variety
of C–X bonds.8 In spite of the great practical importance of the
transition metal chemistry of organoboranes, little is known about
the mechanisms of reactions of organoboron compounds and
electrophilic transition metals. In the transition metal-mediated
C–C coupling reactions of organoboron compounds it is usually
presumed that the boron atom is only involved in the transmetalla-
tion of a boron-bound hydrocarbyl to an electrophilic metal whereas
the high-valent transition metal center is solely responsible for the
product-forming step.3,8 Previously, we have characterized the direc-
tion and the nature of the migration of the methyl and phenyl
Scheme 1 Oxidatively induced B-to-PtIV hydrocarbyl transfer.9–11
Chart 1 New borate ligand and derived compounds.
The new borate ligand and the derived anionic PtII complexes
groups R between an anionic boron atom of the dimethyl- in the form of their sodium salts, Na[2]–Na[5], were prepared
ꢀ
and diphenyldi(2-pyridyl)borates R2B(C5H4N)2 (L) and the nearby using standard synthetic techniques and fully characterized. The
electrophilic PtIVR02 fragment (R0 = Me, Ph) which can result, in chloride salt of the doubly protonated dipyridylborate, H[1]ꢁHCl,
particular, from the oxidation with O2 of the organoplatinum(II) was also characterized by single crystal X-ray diffraction.
complexes Na[LPtIIR02] (see examples in Scheme 1).9–11 In this work
The reactivity of the new organoplatinum(II) organoborates
we report that the anionic alkylborate 1ꢀ present in the PtII com- Na[2]–Na[5] toward O2 in methanolic solution was studied next.
plexes Na[2]–Na[5] (Chart 1) can be involved in a Pt-mediated double We anticipated that compounds Na[2]–Na[5] bearing one or two
oxidative cleavage of the bridgehead alkylborate C(sp3)–H bonds hydrocarbyl groups at the PtII center would be reactive toward O2 in
accompanied by an unprecedented CQC coupling at the boron methanol.9,10,12–14 We also expected that the 9-borabicyclo[3.3.1]-
center, with O2 as the sole oxidant. According to our mechanistic nonane (9-BBN) fragment present in these complexes would
proposal, the C–C/CQC coupling occurs at the boron center.
resist B-to-PtIV hydrocarbyl migration due to the constraints
imposed by its bicyclic structure.
While the platinum-free compound Na[1] dissolved in
methanol showed no change under 1 atm O2 after 4 days at
20 1C, the aerobic oxidation of the hydrocarbyl methoxo PtII
complexes Na[4] and Na[5] in methanol-d4 was facile and took
Department of Chemistry and Biochemistry, University of Maryland, College Park,
MD 20742, USA. E-mail: avederni@umd.edu
† Electronic supplementary information (ESI) available: The experimental details.
CCDC 932583–932586. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c3cc47445c
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were observed in the reaction of O2 with the diphenylplatinum(II)
complex Na[3] in methanol.‡
The stepwise character of the oxidative CQC coupling in
Scheme 3 was proven in the following experiments. When
reaction of Na[3] with O2 was performed under conditions of
starvation of oxygen using only 0.5 equiv. O2 the cleavage of one
of the ligand 1ꢀ bridgehead C–H bonds and the selective
formation of a C–C coupled product Na[8] (94% NMR yield),
the diphenylplatinum(II) analog of Na[7], was observed.
Scheme 2 Oxidative C–C coupling of Na[4] and Na[5].
1
The identity of Na[8] was confirmed using H NMR spectro-
scopy and ESI-MS. A small amount of 11 (6% NMR yield) and the
equivalent quantity of 9 were also detected, presumably because
of a non-ideal control of the reaction stoichiometry. When more
oxygen was admitted to the reaction mixture containing Na[8],
all of the complex was cleanly converted to 9 and 11.
A similar reaction of O2 with the dimethylplatinum(II) complex
Na[2] was more challenging to characterize. A low temperature
of ꢀ60 1C was required to avoid fast protonolysis of Na[2] as well as
the protonolysis of the expected intermediate Na[7]. After warming
from ꢀ60 1C up to 20 1C a solution of the dimethylplatinum(II)
complex Na[2] in MeOH while bubbling slowly O2 through it the
formation of 9 and 10 (88% isolated yield), the dimethyl analog
of 11, was observed. No methane was detected.
An oxidative C–C coupling of lithium salts containing B,B-dialkyl-9-
boratobicyclo[3.3.1]nonane anions to form bicyclo[3.3.0]octylboranes
using acyl chlorides as oxidants was reported in the mid-1970s but it
was never explored mechanistically.16 To the best of our knowledge,
the examples of the platinum-mediated oxidative C–H cleavage are
very rare17 and the CQC coupling in Scheme 3 is unique.8,18 These
considerations motivated us to carry out some mechanistic tests. To
probe if free alkyl radicals are involved in the oxidative C–H bond
cleavage/C–C coupling of Na[2]–Na[5] (Scheme 4, mechanism a), the
oxidation with O2 of two representative compounds, Na[3] and Na[4],
was performed in the presence of 2 equiv. of a radical trap TEMPO.
These reactions produced no O-alkyl TEMPO derivatives that might
have resulted from the recombination of TEMPO and transient free
alkyl species. The reactions also were not inhibited. These facts do not
support mechanism a. In addition, no H/D exchange at the ligand
bridgehead carbon atoms in Na[3] dissolved in CD3OD was observed
Fig. 1 ORTEP drawings (50% probability ellipsoids) of: (a) the anionic
fragment of Na[6], (b) complex 11. Hydrogen atoms are omitted for clarity.
less than 5 min under the conditions indicated above (Scheme 2).
Both reactions led to the formation of a platinum-containing product
Na[6] in >96% NMR yield, H2O and a hydrocarbon, methane or
benzene, respectively, identified by means of NMR spectroscopy. The
identity of Na[6] was confirmed using the Electro-Spray Ionization
Mass-Spectrometry (ESI-MS) and single crystal X-ray diffraction. This
characterization revealed that the 1,5-cyclooctanediyl fragment
present in Na[4] and Na[5] is converted to the bicyclo[3.3.0]octyl
group (Fig. 1a) as a result of the cleavage of one of the ligand
bridgehead C–H bonds and an intramolecular C(sp3)–C(sp3)
coupling. The dimethoxoplatinum(II) complex Na[6] is completely
inert under 1 atm O2 in methanol at 20 1C for at least 2 days.
Further exploration of the unusual aerobic C–C coupling reactivity
of the PtII complexes derived from Na[1] involved the diphenyl-
platinum(II) complex Na[3]. The reaction of Na[3] under 1 atm O2 in
methanol was complete after 10 min at 20 1C and led to a virtually
1
quantitative formation of the bicycloolefin 9, as confirmed using H
and 13C NMR spectroscopy and GS-MS, dimethoxodi(2-pyridyl)borato15
diphenylplatinum(IV) hydroxo complex 11 (isolated yield 95%) and two
moles of H2O (Scheme 3). In contrast to the monophenyl complex
Na[5], no benzene was formed in this reaction. Solid reaction residues
were strongly alkaline confirming the formation of sodium methoxide.
The identity of 11 was proven using ESI-MS and single crystal X-ray
diffraction (Fig. 1b). Hence, a double oxidative cleavage of the ligand 1ꢀ
bridgehead C–H bonds and an intramolecular oxidative CQC coupling
Scheme 3 Aerobic CQC coupling of Na[2] and Na[3].
Scheme 4 Possible mechanisms of the C–H bond cleavage.
Chem. Commun., 2014, 50, 5376--5378 | 5377
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for at least 3 days so arguing against an initial deprotonation of the
As opposed to the reaction of Na[4] and Na[5] with O2, the reaction
ligand bridgehead C–H bonds and subsequent oxidation of the of Na[2] and Na[3] with O2 does not stop at the formation of the C–C
resulting carbanions (Scheme 4, mechanism b). coupled product. The dihydrocarbyl platinum(II) complexes Na[7] and
The C–H bond cleavage/C–C coupling mechanism (Scheme 4, Na[8] are much more reducing compared to Na[6] and can be
mechanism c) involving the formation of a PtIV center and the involved in another O2 activation/hydride abstraction-oxidative C–C
hydride abstraction by PtIV allows us to account for the available coupling/O2 activation reaction sequence to produce 9 along with 10
observations as explained below. First, the oxidation of PtII hydro- and 11, respectively (Scheme 5). In support of the mechanism
carbyl complexes with O2 and H2O2‡ in hydroxylic solvents produces in Scheme 5, Na[15] was produced in situ by reacting the
PtIV hydroxo species with an axial hydroxo ligand, such as 12, with the diphenylplatinum(IV) complex 11 with NaBH4 or NaBH(OMe)3
oxygen atom of the hydroxo ligand originating from the oxidant.12–14 in methanol. The solution of Na[15] then reacted with O2 to
Indeed, we observed the formation of the 18O-labeled product 11-18O form 11 at a fast rate in a quantitative NMR yield.
in the reaction of Na[3] and 18O2 (Scheme 3). Second, the intermedi-
The observed reactivity suggests that 11 may be a good catalyst for
ates 12 derived from the complexes Na[2]–Na[5] transfer the hydride the oxidation with O2 of some hydride donors. Indeed, the oxidation
anion from a bridgehead C–H bond of the 9-BBN fragment to the PtIV with O2 of 0.7 M Na[BH(OMe)3] or NaBH4 in methanol–NaOMe
center to form PtIV hydrides 12a and then 12b. Third, the hydride solutions is catalyzed efficiently by 0.5 mol% 11 with a turnover
ligand at the PtIV atom of 12b is oxidatively coupled with either the frequency of B170 hꢀ1 and B216 hꢀ1, respectively, at 20 1C.
hydrocarbyl R to produce methane or benzene along with Na[6] or
In summary, we have characterized a rare oxidative Pt-mediated
with the OH ligand to form H2O along with the hydrocarbyl PtII C(sp3)–H bond cleavage accompanied by an unprecedented CQC
complex Na[7] or Na[8]. Overall, the reactions of Na[4] and Na[5] with coupling at the boron center of the platinum(II) complexes Na[2]–
O2 lead to Na[6], H2O and R–H as the final products (Scheme 4, Na[5]. Up to three O2 activation steps at a single PtII atom may be
mechanism c); the dimethoxoplatinum(II) complex Na[6] is too involved in these facile transformations. Further study of this novel
electron-poor to further react with O2.
In support of the C-to-PtIV hydride transfer mechanism c, the oxidation and CQC coupling of organoboron and similar compounds.
origin of the protium atom in the hydrocarbon R–H resulting from We thank the National Science Foundation (CHE-0614798,
system might lead to some practical applications utilizing O2 in catalytic
the oxidation of complexes Na[4] and Na[5] (Scheme 2) was revealed CHE-1112019) for the financial support of this work.
using the partially deuterated compounds Na[4-d6] and Na[5-d8].
Notes and references
‡ The use of H2O2 instead of O2 in the reaction with Na[3] also leads to a
The oxidation of the labeled complexes in methanol-d4 leads to the
formation of CD3H or C6D5H, respectively, along with Na[6-d9]. The
only source of protium in the reaction mixtures above that could be
involved in the formation of CD3H and C6D5H is the 9-BBN
fragment. The facile hydride abstraction from the 9-BBN moiety
may be due to the rigid structure of 12 that positions the hydrogen
atom of one ligand bridgehead C–H bond in the close proximity
of the PtIV center, so helping to diminish the reaction activation
barrier. Some metal complexes derived from 1,5-cyclooctanediylbis-
(1-pyrazolyl)borate, a dipyrazolyl analogue of 1ꢀ having the 9-BBN
framework, were reported to have an enhanced C–H agostic inter-
actions of their bridgehead C–H bonds with the metal.19 In support
of this hypothesis, a stable trimethylplatinum(IV) complex 13,
[1]PtMe3, was prepared as a model of 12. Complex 13 was character-
ized using single crystal X-ray diffraction and found to have a short
1.99 Å agostic (d-C)H–PtIV bond.§
fast formation of 9 and 11 (90% NMR yield).
§ In 1H NMR spectra (CD2Cl2) the agostic proton resonance is
at ꢀ3.36 ppm with the large coupling constant JPt–H = 208 Hz.
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Scheme 5 Proposed mechanism of the oxidative C–H bond cleavage/
CQC coupling of Na[7] (R = Me) and Na[8] (R = Ph).
5378 | Chem. Commun., 2014, 50, 5376--5378
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